Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning

[Mathieu Desnoyers]

On 2023-10-26 10:06, Mateusz Guzik wrote:
> On 10/26/23, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > On Thu, Oct 26, 2023 at 05:46:21PM +0530, Abhinav Singh wrote:
> > > This patch fixes the warning about directly dereferencing a pointer
> > > tagged with __rcu annotation.
> > >
> > > Dereferencing the pointers tagged with __rcu directly should
> > > always be avoided according to the docs. There is a rcu helper
> > > functions rcu_dereference(...) to use when dereferencing a __rcu
> > > pointer. This functions returns the non __rcu tagged pointer which
> > > can be dereferenced just like a normal pointers.
> > >
> > > Signed-off-by: Abhinav Singh <singhabhinav9051571833@xxxxxxxxx>
> >
> > Well yes but these need to be called under rcu_read_lock.
> > Who does it here?
> > If no one then maybe you found an actual bug and we need to
> > fix it not paper over it.
>
> There is no bug here.
>
> p is the newly created thread, ->real_cred was initialized just prior
> to this code and there is nobody to whack the creds from under it.
>
> Second bit in the patch changes one real_parent deref, but leaves 2
> others just above it. Once more no bug since the entire thing happens
> under tasklist_lock, but the patch should either sort all these cases
> or none.

Drive-by comment: perhaps use rcu_dereference_protected() ?

Thanks,

Mathieu

> I think it would help if the submitter had shown warnings they see.

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com